1. Field of the Invention
The invention relates to a monitor unit for monitoring an optical waveguide and to a communications system incorporating such a monitor unit. The invention relates particularly, but not exclusively, to monitor units and communications systems in which the optical waveguide comprises an optical fibre.
2. Description of the Related Art
It is well known in the art to form a communications system having a communications optical fibre, a transmitter for transmitting data in the form of an optical signal along the communications optical fibre, and a receiver for receiving the signal. A disadvantage of a transmission system of this type is that it does not provide an inherently secure means of transmission in that the process of physically interfering with the communications system is becoming increasingly simple to perform. There is a need, therefore, for means for monitoring communications systems for such interference.
Also, it is often desired to monitor the waveguide for chances caused by other means. For example a sensor may be connected to the waveguide which physically affects the waveguide in response to a chance in some parameter, for example temperature or pressure. In other cases the waveguide may teach or change in response to its environment, for example temperature changes or splice ageing, and it is desired to monitor these changes.
One known way of monitoring a communications system is to apply a corrugated attachment to the outer surface of a fibre at a point at which physical interference of the system is anticipated or at the location of the sensor, the sensor mechanism applying pressure to the corrugations. When the fibre is interfered with or it is activated by the sensor, the corrugations are caused to press onto the outer surface of the fibre thereby causing the loss of some of the signal being received. This reduces the total optical energy of the signal so by monitoring the total optical energy physical interference may be detected. Similarly mode transfers can be caused by environmental influences such as temperature or ground movements acting on the waveguide directly.